The present invention relates to industrial chemical processes in which a processing chemical initially in the form of a liquid is converted to the vapor state, in which state it is conveyed to a processing station.
In many industrial chemical processes, a processing chemical is stored in a container in a liquid state and the supply of processing chemical in the container is gradually exhausted as the liquid is converted to a vapor and expelled from the container. According to one technique currently employed for converting such a processing chemical to a vapor, a fill gas is introduced into the container via an inlet opening at the top of the container. This fill gas creates a high pressure region above the processing chemical. The container is also provided with an outlet line having an outlet pipe which is immersed in the processing chemical and which leads, via a filter and appropriate valves, to a flow control device, such as a liquid mass flow controller. The flow control device delivers the processing chemical to a vaporizer, together with a carrier gas. Within the vaporizer, the carrier gas mixes with the processing chemical to produce a vapor which is then delivered to the processing station. The processing station may be composed of a process chamber whose interior is maintained at a low pressure that acts to draw the vapor from the vaporizer.
In apparatus of the type described above, the vaporizer normally contains orifices of very small size, in the range of 0.001 to 0.030 inch in diameter. These orifices can be easily clogged. Although it is common practice to dispose a filter in the flow path which conveys the processing chemical to the vaporizer, and such a filter effectively prevents particles already present in the processing chemical from reaching the vaporizer, the vaporizer is nevertheless subject to clogging as a result of thermal decomposition of the processing chemical in the vaporizer. The resulting thermal decomposition products will often plate the orifices in the vaporizer, resulting in clogging of those orifices. Such thermal decomposition may occur when all operating conditions are within their normal range, but will occur at an increased rate if the temperature within the vaporizer is outside of a desired range.
Eventually, the vaporizer can become clogged to such a degree that an acceptable vapor flow is no longer being produced. However, when an unacceptably low vapor flow rate is first noted, the source of the problem may not be immediately apparent. For example, the problem may be caused by a failure or a blockage of any component in the vapor delivery system or the processing chemical delivery system. Therefore, when such a decrease in the vapor flow rate has been observed, the system often must be shut down, typically for many hours, to allow the source of the problem to be identified, and the necessary repair or replacement to be made.
If it is found that the problem is caused by vaporizer clogging, it is usually necessary to undertake a long and complicated process to properly cool down and purge the lines associated with the vaporizer in order to avoid creation of conditions which could be hazardous to the operating personnel and in order to prevent additional contamination of the hardware. The resulting apparatus downtime will, of course, have an adverse effect on production throughput.
A number of techniques for monitoring the operating condition of a vaporizer in such apparatus have been proposed and used in practice. Each of these techniques possesses certain inherent disadvantages.
One such technique involves sensing of the vapor pressure downstream of the vaporizer. If this pressure drops, it could be the result of vaporizer clogging. However, this approach typically requires that a pressure sensor be placed in contact with the reactive vapor. Exposure to the reactive vapor can be corrosive to the sensor components. Furthermore, it may be necessary to heat the sensor, which can reduce its operating life. In addition, in order for the sensor to properly monitor the downstream vapor pressure, it often must be located in such a manner that it can have an adverse effect on the fluid conductance in the vapor flow path. If the fluid flow conductance is reduced, the result can be lower deposition rates. Furthermore, such sensors can introduce dead-legs which are in communication with the vapor flow path. The presence of dead-legs leads usually complicates purging resulting in prolonged purge cycles during line maintenance.
In apparatus of the type here under consideration, the carrier gas is typically delivered to the vaporizer by a mass flow controller. The operation of such a controller can be monitored and a detected controller fault condition may be due to severe blockage downstream of the controller. However, it is often the case that only a severe blockage downstream of the controller can be relied upon to produce a fault indication since the controller itself operates in a manner to compensate for partial obstructions. In addition, faults occurring within the controllers themselves are all too frequent, whether or not the vaporizer is obstructed. Therefore, monitoring of the controller status usually does not provide a particularly useful indication of vaporizer problems.
Similarly, it is known to monitor the liquid mass flow controller which is delivering processing chemical to the vaporizer. Here again, if an indication of a liquid mass flow controller fault is due to blockage of the vaporizer, such indication often will not be produced until a severe blockage has occurred. In addition, an indication of a liquid mass flow controller fault can be due to a number of other causes, including filter clogging. Therefore, monitoring of liquid mass flow controller faults to identify vaporizer clogging has drawbacks similar to those of monitoring the carrier gas mass flow controller.
When the vapor is delivered to a processing station in order to form a layer, or film, on a workpiece, or substrate, inspection of the resulting film can provide an indication of whether vapor has been delivered to the processing station at the desired rate. Thus, although the vaporizer may be only partially clogged, some property, such as thickness, of the resulting film can be outside of specifications. However, such film defect can be due to a number of causes and therefore often does not allow vaporizer clogging to be pinpointed with any degree of reliability. Furthermore, because each semiconductor substrate can be worth tens of thousands of dollars, it is highly desirable to detect vaporizer clogging before a substrate is ruined.